1. Field of the Invention
This invention relates to a magnet assembly and a voice coil motor using such magnet assembly, more particularly, of a magnet assembly and a voice coil motor for use with such equipment as the magnetic head positioning actuator of a hard disc drive unit.
2. Related art
In general, in the field of voice coil motors for positioning of magnetic heads, featuring the combined use of a magnetic circuit and a moving coil, various types of voice coil motors have already been developed having various magnetic circuit structures, magnetic characteristics and moving coil shapes.
FIGS. 14 to 16 show an example of conventional voice coil motors wherein the exemplified voice coil motor is provided with a roof-shaped yoke 2 having a circular arc shape at its lower surface and in a roof shape at its upper surface, wherein beneath the yoke 2, a yoke 6 of circular arc shape is provided with both ends directed upwards for connection to the roof-shaped yoke 2, thereby forming a required gap 4 between the yokes 2 and 6.
Meanwhile, the terms up and down, above and below or over and beneath are merely to be used for explanatory purposes in relation to the drawings but not to indicate relative positions in the actual structure.
Beneath the roof-shaped yoke 2, or in the central part of the circular arc surface facing the gap 4, two units of upper magnetic pieces 8A and 8B having an arc shape are adhered, on the upper surface of circular arc-shaped yoke 6, or in the central part of the circular arc surface facing the gap 4, two units having a lower magnet 10A and 10B of circular arc shape of smaller radii than the radii of upper magnets 8A and 8B, are adhered in concentricity with the upper magnet pieces 8A and 8B.
The pairs of magnet pieces are magnetized in opposing directions. For example, as FIG. 16 shows, with the right-hand upper and lower magnet pieces 8A and 10A, the magnetizing direction 12 is directed toward the outside of the radial direction of the circular arc, while with the left-hand upper and lower magnet pieces 8B and 10B, the magnetizing direction 14 is directed toward the inside of the radial direction of the circular arc.
As FIGS. 14 and 15 show, in gap 4 formed by the upper and lower magnet pieces, a coil 18 fixed to the top end of an arm 16 which can swing around the center 0 of the circular arc as the supporting point is so installed to move in a circular arc locus through the gap 4.
To the other end of the arm 16, for example, a magnetic head 20 is installed so that when the arm 16 swings, the magnetic head 20 moves in the radial direction of turning magnetic disc 22.
A magnetic disc drive unit must have a higher thrust and, at the same time, flatter thrust-displacement characteristics, otherwise, if thrust is not constant against displacement, dispersion occurs with access time.
The magnet pieces 8A, 8B, 10A and 10B preferably include magnet materials with higher magnetic properties such as Sm.sub.2 Co.sub.17 or Nd-Fe-B sintered magnet pieces. Such sintered magnet pieces are produced by filling, for example powdered alloy into a cavity 30 shaped in sector form, or a circular arc, provided on the surface of a mold block 28 as shown in FIG. 17, and by press-forming the alloy powder filled into the cavity 30 using a push rod, which is not indicated in FIG. 17, before sintering the thus press-formed alloy powder.
During the sintering process, the magnet materials are formed with their magnetic orientation in a specified direction by magnetizing, for example, to a magnetic field 32 directed in the horizontal direction to provide the required orientation in the magnetic field of the magnetic materials.
With a magnet piece thus produced, a phenomenon is known to occur where the magnetic flux density in the center portion is higher than that at both ends of the magnet piece. If these magnet pieces of dispersed magnetic flux density are used, the thrust becomes larger in sections with higher magnetic flux density and lower in sections with lower magnetic flux density and constant thrust cannot be maintained against displacement.
The degree of magnetic flux density is proportional to the magnet's cubic volume with a uniform material magnet. In order to obtain uniform magnetic flux density throughout all the magnet sections, a method has been adopted whereby the thickness at the central portion of a magnet where magnetic flux density becomes higher, is made thinner.
FIG. 16 shows an example of the method wherein the upper arc surfaces of the lower magnet pieces 10A and 10B are cut and removed to form flat surfaces of equal length (L1=L2) from the center planes 24A and 24B , which equally divide the lower magnet pieces 10A and 10B. 26A and 26B represent the cut planes.
Nevertheless, even if such magnet pieces are made thinner at the central portion and used as the magnetic circuit of a voice coil motor as shown in FIG. 14 and FIG. 15, it has not been possible to move the magnetic head 20 with a constant thrust throughout the swinging range of the coil 18 of the motor and of the arm 16.